[Devel] Re: [PATCH v4 06/14] memcg: kmem controller infrastructure

[Michal Hocko]

On Fri 12-10-12 12:44:57, Glauber Costa wrote:
> On 10/12/2012 12:39 PM, Michal Hocko wrote:
> > On Fri 12-10-12 11:45:46, Glauber Costa wrote:
> >> On 10/11/2012 04:42 PM, Michal Hocko wrote:
> >>> On Mon 08-10-12 14:06:12, Glauber Costa wrote:
> > [...]
> >>>> +	/*
> >>>> +	 * Conditions under which we can wait for the oom_killer.
> >>>> +	 * __GFP_NORETRY should be masked by __mem_cgroup_try_charge,
> >>>> +	 * but there is no harm in being explicit here
> >>>> +	 */
> >>>> +	may_oom = (gfp & __GFP_WAIT) && !(gfp & __GFP_NORETRY);
> >>>
> >>> Well we _have to_ check __GFP_NORETRY here because if we don't then we
> >>> can end up in OOM. mem_cgroup_do_charge returns CHARGE_NOMEM for
> >>> __GFP_NORETRY (without doing any reclaim) and of oom==true we decrement
> >>> oom retries counter and eventually hit OOM killer. So the comment is
> >>> misleading.
> >>
> >> I will update. What i understood from your last message is that we don't
> >> really need to, because try_charge will do it.
> > 
> > IIRC I just said it couldn't happen before because migration doesn't go
> > through charge and thp disable oom by default.
> > 
> 
> I had it changed to:
> 
>         /*
>          * Conditions under which we can wait for the oom_killer.
>          * We have to be able to wait, but also, if we can't retry,
>          * we obviously shouldn't go mess with oom.
>          */
>         may_oom = (gfp & __GFP_WAIT) && !(gfp & __GFP_NORETRY);

OK

> 
> >>>> +
> >>>> +	_memcg = memcg;
> >>>> +	ret = __mem_cgroup_try_charge(NULL, gfp, size >> PAGE_SHIFT,
> >>>> +				      &_memcg, may_oom);
> >>>> +
> >>>> +	if (!ret) {
> >>>> +		ret = res_counter_charge(&memcg->kmem, size, &fail_res);
> >>>
> >>> Now that I'm thinking about the charging ordering we should charge the
> >>> kmem first because we would like to hit kmem limit before we hit u+k
> >>> limit, don't we. 
> >>> Say that you have kmem limit 10M and the total limit 50M. Current `u'
> >>> would be 40M and this charge would cause kmem to hit the `k' limit. I
> >>> think we should fail to charge kmem before we go to u+k and potentially
> >>> reclaim/oom.
> >>> Or has this been alredy discussed and I just do not remember?
> >>>
> >> This has never been discussed as far as I remember. We charged u first
> >> since day0, and you are so far the first one to raise it...
> >>
> >> One of the things in favor of charging 'u' first is that
> >> mem_cgroup_try_charge is already equipped to make a lot of decisions,
> >> like when to allow reclaim, when to bypass charges, and it would be good
> >> if we can reuse all that.
> > 
> > Hmm, I think that we should prevent from those decisions if kmem charge
> > would fail anyway (especially now when we do not have targeted slab
> > reclaim).
> >
> 
> Let's revisit this discussion when we do have targeted reclaim. For now,
> I'll agree that charging kmem first would be acceptable.
> 
> This will only make a difference when K < U anyway.

Yes and it should work as advertised (aka hit the k limit first).

You can stick my Acked-by then.
-- 
Michal Hocko
SUSE Labs